docs: def schwefelmult(self, x, pen_fac=1e4):
          """multimodal Schwefel function with domain -500..500"""
          y = [x] if isscalar(x[0]) else x
          N = len(y[0])
          f = array([418.9829 * N - 1.27275661e-5 * N - sum(x * np.sin(np.abs(x)**0.5))
                  + pen_fac * sum((abs(x) > 500) * (abs(x) - 500)**2) for x in y])
          return f if len(f) > 1 else f[0]